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Old 03-09-2011, 01:47 PM   #1
AVANTI R5
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Default Gas Mileage Myths



We identify and discredit six misconceptions about fuel economy.




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Do Americans care about fuel economy as the average price for a gallon of gasoline hovers at $3.50 and tops $4 in some parts of the country? You bet they do, though they also have a fair number of misconceptions about how to squeeze a few more miles out of every drop.
The Consumer Federation of America's most recent survey says that if we had a 50 mile-per-gallon car fleet today, we'd save more oil than there the entire proven reserves in the entire Gulf of Mexico. And people care about that. According to Jack Gillis, author of The Car Book and a CFA spokesman, 87 percent of respondents said it is "important that the country reduce its consumption of oil," and 54 percent said it is "very important."

The Most Fuel-Efficient 2011 Cars


Toyota Prius
n amazing 65 percent support a mandated transition to a 50-mpg fuel economy standard by 2025-a figure that presumably includes even some Tea Party supporters. That's a tough standard, some 15 mpg better than the ambitious goal set by the Obama Administration (35 mpg by 2016). (Though the Prius, the most fuel-efficient car on the road, gets 50 mpg today.)

8 Facts and Myths About Warming Up Your Car in Winter

"The expectations of American consumers are reasonable and achievable," Gillis said in a conference call." He said that using such available and on-the-shelf technologies as cylinder deactivation and engine cutoff when stopped (the so-called "mild hybrid" popular in Europe) we could make five to 10 percent economy improvements.

10 No-Brainer Ways to Save Money on Gas

CFA says that Asian carmakers, compared to the U.S. competition, are offering twice as many vehicles with 30 mpg or better. "It's shocking that so few of today's cars get more than 30 mpg," he said. I agree. I'm test-driving a $17,000 Hyundai Elantra Blue that gets 35 mpg on the highway, and that kind of economy is routine for Asian carmakers.
Mark Cooper, CFA's research director, said that in five years of the group's polling, the public's views have stayed remarkably consistent: They want less dependence on Middle Eastern oil and higher fuel-economy standards.

7 New Electric Cars Available in 2011

Cooper pointed out that Gulf oil is a big player when it comes to U.S. reserves, but is "inconsequential" in terms of world supply. The U.S. has just three percent of world oil, though most people think we have a much bigger piece of the pie. When informed of this unpleasant fact, Cooper said, the percentage that thinks it's "very important" to reduce oil dependence goes up significantly (from 54 percent to 68).

People care about fuel economy, but they're misinformed about how to actually achieve it. The federal government's fueleconomy.gov site (very useful to check cars' mpg) just published the "Top Ten Misconceptions about Fuel Economy." Here are a few (the ones I like).


7 Of the Best Fuel-Efficient SUV Crossovers

It takes more fuel to start a vehicle than it does to let it idle.
People are really confused about this one, and will leave a car idling for half an hour rather than turn it off and restart. Some kids I know started an anti-idling campaign in the suburbs and are shaming parents into shutting down their cars. Idling uses a quarter to a half gallon of fuel in an hour (costing you one to two cents a minute). Unless you're stalled in traffic, turn off the car when stopped for more a few minutes.

Vehicles need to be warmed up before they're driven.
That is a long-outdated notion. Today's cars are fine being driven off seconds after they're started.



With the proper maintenance a 10- or 15-year-old car such as this 2000 Cadillac Seville should have like-new mileage.




As a vehicle ages, its fuel economy decreases significantly.

Not true. As long as it's maintained, a 10- or 15-year-old car should have like-new mileage. The key thing is maintenance: An out-of-tune car will definitely start to decline mileage-wise.

Replacing your air filter helps your car run efficiently.

Another outdated claim, dating back to the pre-1976 carburetor days. Fuel-injection engines don't get economy benefits from a clean air filter.

Aftermarket additives and devices can dramatically improve your fuel economy.

As readers of my story on The Blade recall, there's not much evidence that these "miracle products" do much more than drain your wallet. Both the Federal Trade Commission and Consumer Reports have weighed in on this. There are no top-secret 100-mpg bolt-ons out there.

Using premium fuel improves fuel economy.

You might as well write a check to BP if you believe this. Only use premium if your car specifies it.
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Old 03-09-2011, 02:55 PM   #2
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Replacing your air filter helps your car run efficiently.

Another outdated claim, dating back to the pre-1976 carburetor days. Fuel-injection engines don't get economy benefits from a clean air filter.
I respectfully disagree with above.
While modern sensors (MAF) and computer will adjust the air/fuel mixture on the fly, the engine that has a dirty filter will have to work harder to suck enought of it into the cylinders. This means that there is more load on the engine to achieve the same result as with clean filter.
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Old 03-09-2011, 02:57 PM   #3
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Originally Posted by rallymaniac View Post
will have to work harder to suck enought of it into the cylinders. This means that there is more load on the engine to achieve the same result as with clean filter.

Suck harder for bigger load? Interesting...
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Old 03-09-2011, 02:59 PM   #4
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Considering the WRX requires premium (91 or 93) gas, I do wonder what cars are required to use the mid grade 89 stuff, and why it's available everywhere.
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Old 03-09-2011, 03:10 PM   #5
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Originally Posted by Yotsuya View Post
Considering the WRX requires premium (91 or 93) gas, I do wonder what cars are required to use the mid grade 89 stuff, and why it's available everywhere.
...except at Costco and Sam's Club
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Old 03-09-2011, 03:11 PM   #6
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i would disagree with air filter claim
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Old 03-09-2011, 03:17 PM   #7
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A fair question: How restricted does an air filter have to be before it starts to reduce an engine's volumetric efficiency enough to produce a measurable reduction in fuel efficiency?

A filter full of dirt will be problematic, but a lightly crudded city filter should go unnoticed.
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Old 03-09-2011, 04:09 PM   #8
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Originally Posted by Yotsuya View Post
Considering the WRX requires premium (91 or 93) gas, I do wonder what cars are required to use the mid grade 89 stuff, and why it's available everywhere.
My '87 Subaru Justy manual calls to use 89.
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Old 03-09-2011, 05:58 PM   #9
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Hm, guess I've never come across a car needing anything other than regular or premium.
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Old 03-09-2011, 06:49 PM   #10
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Originally Posted by rallymaniac View Post
I respectfully disagree with above.
While modern sensors (MAF) and computer will adjust the air/fuel mixture on the fly, the engine that has a dirty filter will have to work harder to suck enought of it into the cylinders. This means that there is more load on the engine to achieve the same result as with clean filter.
Gasoline engines are throttled anyway. The whole point of the throttle plate is to creat an intake restriction. If you never fully open the throttle plate (floor it), there is technically no reduction in efficency of the engine. It just requires the pedal to be pushed a bit farther down to achieve the same restriction. If you have a really cloged filter its effect is similar to not being able to open the throttle very much. I am sure this varies from car to car and how the fuel is metered. But fundamentally speaking an intake restriction doesnt really cause a loss of efficency for part throttle. It does create a peak power loss though, because it is an artificially partially closed throttle.

A diesel engine however would gain efficency as there is no air throttle/
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Old 03-09-2011, 07:12 PM   #11
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Originally Posted by rallymaniac View Post
I respectfully disagree with above.
While modern sensors (MAF) and computer will adjust the air/fuel mixture on the fly, the engine that has a dirty filter will have to work harder to suck enought of it into the cylinders. This means that there is more load on the engine to achieve the same result as with clean filter.
No, that's not what it means. Think about it for a second.

Gas engines have throttles, and whether you restrict the intake air flow at the throttle or at the air filter makes no difference.

If you lose 50 mbar across the air filter, you can simply open the throttle a bit further, get the same amount of air into the cylinders and burn the same amount of fuel.

edit: aschen was quicker.
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Old 03-09-2011, 11:07 PM   #12
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Originally Posted by bal00 View Post
No, that's not what it means. Think about it for a second.

Gas engines have throttles, and whether you restrict the intake air flow at the throttle or at the air filter makes no difference.

If you lose 50 mbar across the air filter, you can simply open the throttle a bit further, get the same amount of air into the cylinders and burn the same amount of fuel.

edit: aschen was quicker.
The more closed the throttle is, the lower the pressure in the intake is, and the greater the pressure differential between the inlet and outlet of the engine. Maintaining a greater pressure differential requires more work, and therefore requires more fuel. The closer to ambient pressure you can get the intake manifold, the less hard the engine has to work. I really hate to use the term "pumping losses" but that's it in a nutshell.

However, as most people have observed after adding a hugeass cone filter to their turbocharged vehicles, the increase in fuel economy and power associated with "high flow" (lower pressure drop) air filter is generally insignificant, as is the reduction with a dirty vs clean filter. There are so many energy losses associated with ICEs and the overall vehicle, that the pumping loss is relatively small fraction of the total energy used.

A more efficient configuration is having no throttle, and controlling volumetric efficiency using valve timing only. If you lower the intake pressure drop you can get the same amount of air to flow into the engine in a shorter time, and reduce the time spent "pumping" against a higher pressure differential between the intake and exhaust manifolds.

VVVVVVV

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Originally Posted by aschen View Post
Even if you throttle with valve timeing you still have "pumping losses" I think for a fixed discplacment engine.
That is correct - you cannot completely eliminate "pumping losses" because engines are air pumps by nature - they cause a lower pressure on the inlet and a higher pressure on the outlet.

Quote:
The way to get around this inherent compromise for spark ignition engine is to have one that can operate in lean burn (throttle with amount of fuel injected) or variable discplacement where the intake volume is always atmospheric but you vary the intake stroke or whatever to adjust the power level.
Exactly, such as on a diesel =) or an as yet to be made truly variable displacement engine =(

Last edited by amanichen; 03-09-2011 at 11:43 PM.
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Old 03-09-2011, 11:27 PM   #13
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Even if you throttle with valve timeing you still have "pumping losses" I think for a fixed discplacment engine. I have to think about it more though. The problem is gasoline engines only work in a narrow air fuel range, so at some point if not at max power, you will need to be working against a vacume.

The way to get around this inherent compromise for spark ignition engine is to have one that can operate in lean burn (throttle with amount of fuel injected) or variable discplacement where the intake volume is always atmospheric but you vary the intake stroke or whatever to adjust the power level.

Either way, for a typical air throttled engine any restriction upstream of the throttle plate isnt doing any harm until the car is floored. In any part throttle condition it is simply changing the position the user needs to put the pedal to.
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Old 03-09-2011, 11:39 PM   #14
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Vehicles need to be warmed up before they're driven.
That is a long-outdated notion. Today's cars are fine being driven off seconds after they're started.
Oh really? I am going to guess these people never started and ran a car at -35 celsius.
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Old 03-10-2011, 06:40 AM   #15
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Originally Posted by amanichen View Post
The more closed the throttle is, the lower the pressure in the intake is, and the greater the pressure differential between the inlet and outlet of the engine. Maintaining a greater pressure differential requires more work, and therefore requires more fuel.
Yes, that's why gas engines are not terribly efficient at part throttle, but where that pressure drop occurs makes no difference.

If you need an engine to produce say 25 hp to maintain a certain speed, you need to burn a certain amount of air and fuel.

If you remove an intake restriction (such as a very dirty air filter) and keep the throttle where it is, the engine will pump more air, inject more fuel and produce more power. That means you now have to close the throttle a bit further to reduce the air flow back to the old level so you can maintain your speed. In other words, the pressure differential will be exactly the same.


If what you're saying was true, you could improve your gas mileage by drilling holes into the throttle plate (removing an intake restriction). In reality this will not help fuel economy at all, it will just change the pedal position needed to produce a certain amount of power.
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Old 03-10-2011, 08:56 AM   #16
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Originally Posted by bal00 View Post
If what you're saying was true, you could improve your gas mileage by drilling holes into the throttle plate (removing an intake restriction). In reality this will not help fuel economy at all, it will just change the pedal position needed to produce a certain amount of power.
You're confusing the operation of a modern fuel injection system with energy required to pump air.

All engines are air pumps, regardless of how much fuel is added. Maintaining a smaller pressure differential across a pump requires less work and less energy. I don't know how else to put it

Again, the energy spent pumping air through the engine is a small fraction of the total energy used by an engine.
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Old 03-10-2011, 09:08 AM   #17
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Originally Posted by amanichen View Post
You're confusing the operation of a modern fuel injection system with energy required to pump air.

All engines are air pumps, regardless of how much fuel is added. Maintaining a smaller pressure differential across a pump requires less work and less energy. I don't know how else to put it

Again, the energy spent pumping air through the engine is a small fraction of the total energy used by an engine.
except gasoline engines are throttled by maintaining this differenctial pressure. at part throttle The engine is unaware if this is created by the filter or the throttle plate.
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Old 03-10-2011, 09:22 AM   #18
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except gasoline engines are throttled by maintaining this differenctial pressure. at part throttle
Part of the energy output of the engine is used to pump air. The lower the pressure differential across the pump, the less energy is required to pump the air. I don't know how else to explain it other than shouting louder and doing something irrational LIKE GOING TO ALL CAPS!!

Rallymaniac had it correct in the first reply to this thread.

Quote:
The engine is unaware if this is created by the filter or the throttle plate.
This is correct. There are several sources of pressure drop on the inlet side of the engine. One of the biggest are the intake valves themselves. They're quite horrible for efficient airflow, but the shape is also dictated by things like needing to seal the cylinder, and minimizing intrusion into the combustion chamber.

Last edited by amanichen; 03-10-2011 at 10:03 AM.
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Old 03-10-2011, 09:34 AM   #19
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Originally Posted by bal00 View Post
No, that's not what it means. Think about it for a second.

Gas engines have throttles, and whether you restrict the intake air flow at the throttle or at the air filter makes no difference.

If you lose 50 mbar across the air filter, you can simply open the throttle a bit further, get the same amount of air into the cylinders and burn the same amount of fuel.

edit: aschen was quicker.
but you spell better, so it washes out.
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Old 03-10-2011, 09:41 AM   #20
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Originally Posted by rallymaniac View Post
I respectfully disagree with above.
While modern sensors (MAF) and computer will adjust the air/fuel mixture on the fly, the engine that has a dirty filter will have to work harder to suck enought of it into the cylinders. This means that there is more load on the engine to achieve the same result as with clean filter.
It states this, but if you click the "10 ways to save gas" link. The very first thing tells you to clean your air filter and improve mileage 10%.....
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Old 03-10-2011, 11:00 AM   #21
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Originally Posted by Imprezive_04 View Post
It states this, but if you click the "10 ways to save gas" link. The very first thing tells you to clean your air filter and improve mileage 10%.....
note how over half the article is dedicated to external links....

that annoyed me to no f'ing end.

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Old 03-10-2011, 11:10 AM   #22
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Originally Posted by amanichen View Post
Part of the energy output of the engine is used to pump air. The lower the pressure differential across the pump, the less energy is required to pump the air. I don't know how else to explain it other than shouting louder and doing something irrational LIKE GOING TO ALL CAPS!!

Rallymaniac had it correct in the first reply to this thread.

This is correct. There are several sources of pressure drop on the inlet side of the engine. One of the biggest are the intake valves themselves. They're quite horrible for efficient airflow, but the shape is also dictated by things like needing to seal the cylinder, and minimizing intrusion into the combustion chamber.

No need to go all caps lets keep this classy. I am fully aware the power loss is proportional to the pressure differential multiplied by the volume flow rate....that is the time differential of the PV work (I has a degree in thermo dynamics).

However read the replies carfully. Throttleing, or creating a pressure differentional is the means you adjust the power level of a gasoline engine. Half power for instance, requires a certain manifold vacumme lets say -.5 bar.

The engine has no idea if this vacume or pressure differential is created at the throttle body or the filter. If you desire -.5psig, the e scenarios below are exactly the same for fuel efficency:

1. No air filter and .5 bar drop at the plate

2. .1bar drop at the filter and .4bar drop at the throttle plat

the ONLY difference in the above 2 scenarios is that the pedal is a little closer to the floorboard for #2. The problem with the cloged filter in our obove scenario is if you floor the car and set the throttle plate flow resistance 2 0, you still have drop at the filter. It limits the max opening of the throttle to -.1bar (actually more because the mass flow rate goes up and the pressure drop at the filter will increase similarly).

See also, a restrictor plate, doesnt apreciably effect the efficency of a race engine if designed well. It effectively limits how far the throttle can be opened.

We get that it takes power to maintain a pressure differential at a flow rate. The point is that you need that vacume anyways to run at part throttle. However you are missing our fundamental argument. Adress this technical instead of resorting to caps........
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Old 03-10-2011, 12:13 PM   #23
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Originally Posted by amanichen View Post
You're confusing the operation of a modern fuel injection system with energy required to pump air.

All engines are air pumps, regardless of how much fuel is added. Maintaining a smaller pressure differential across a pump requires less work and less energy. I don't know how else to put it

Again, the energy spent pumping air through the engine is a small fraction of the total energy used by an engine.
I'm not confusing anything.

If you reduce the overall pressure differential by getting rid of on an intake restriction (such as a dirty air filter), the driver will simply lift the throttle and restore the original pressure differential. That's how part throttle operation works.


This it how the picture looks with a dirty air filter:

Atmosphere(1bar) --> dirty air filter(-0.1bar) --> partially open throttle (-0.2bar) --> engine (0.7bar abs.) = 35 hp output

Now you replce the dirty filter with a clean one:

Atmosphere(1bar) --> clean air filter(-0.0bar) --> partially open throttle (-0.2bar) --> engine (0.8bar abs.) = 45 hp output

Now the driver would go "wtf, I didn't ask for 45 hp" and lift the throttle because the driving situation only requires 35 hp, and it would look like this:

Atmosphere(1bar) --> clean air filter(-0.0bar) --> partially open throttle (-0.3bar) --> engine (0.7bar abs.) = 35 hp output


The overall pressure differential is exactly the same as it was with the dirty filter, and so is the air flow, the fuel consumption and the power output.
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Old 03-10-2011, 12:21 PM   #24
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Doesn't it also depend on what fueling mode he's in?

If he's running off the tables, and increase in throttle % (to account for a dirty filter) is going to result in an increase IDC.
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Old 03-10-2011, 12:35 PM   #25
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Doesn't it also depend on what fueling mode he's in?

If he's running off the tables, and increase in throttle % (to account for a dirty filter) is going to result in an increase IDC.
Even in open loop mode most ECUs will still look at manifold pressure or mass airflow. Only very basic 'Alpha/N' systems used on some motorcycles and race engines would increase fueling based on the throttle angle alone, without considering the MAF or MAP values.

These engines usually have extremely aggressive cams that would make the MAP signal unreliable, and part throttle driveability/emissions are not important for race engines.
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